In the 1930's, Edwin Hubble discoveried that all galaxies have a positive
redshift. In other words, all
galaxies were receding from the Milky Way.

As it was later discoveried, the higher the redshift of an object, the
farther away it is (Hubble's law). By the 1960's, the farthest objects
detected were quasars.

The early radio surveys of the sky (in addition to discovering radio
galaxies) also detected a number of radio sources that were called
``quasistellar'' radio sources, because, like stars, they were unresolved
in visible images. The term ``QUASistellAR'' was shortened to quasar, the
name by which this class of objects has since been called.

The most distant quasars are approximately 13 billion light years, away.
To be seen at such immense distances, quasars must be very luminous --
even more luminous than a bright galaxy. Quasars have luminosities which
lie in the range L = 10 to 10,000 times the luminosity of the Milky Way
galaxy, 25 billion times the luminosity of the Sun. Thus, even the dimmest
of the quasars are as bright as the brightest galaxies the vicinity of the
Milky Way. The luminosity of quasars can vary significantly on timescales
of only a week. Thus, the bulk of the immense luminosity of a quasar must
be coming from a region only one light-week (1200 A.U.) across!

Quasars have a decidedly non-thermal spectrum: they are luminous in the
X-rays, ultraviolet, visible, infrared, and radio bands. They have about
the same power at all of the wavelengths down to the microwave wavelengths
(shortwave radio wavelengths). The spectrum looks like the synchrotron
radiation from charged particles spiralling around magnetic field lines at
nearly the speed of light.

Quasars tend to be found at great distances from us; there are no nearby
quasars. When we look at quasars, we see them as they were billions of
years ago. The number of them increases at greater distances, so that must
mean they were more common long ago. The number of quasars peaks at a time
when the universe was about 20% of its current age. Back then the galaxies
were closer together and collisions were more common than today. Also, the
galaxies had more gas that had not been incorporated into stars yet. The
number of quasars was hundreds of times greater than the time closer to
the present. At very great distances the number of quasars drops off. The
light from the most distant quasars are from a time in the universe before
most of the galaxies had formed, so fewer quasars could be created.

Astronomers are beginning to find the inactive supermassive black holes in
some galaxies. In most galaxies the central black hole would have been
smaller than the billions of solar mass black holes for quasars. This is
why the less energetic active galaxies are more common than quasars. Our
galaxy harbors a supermassive black hole in its core that has a mass of
``only'' 2.5 million solar masses. Astronomers are studying the cores of
other normal galaxies to see if there are any signs of supermassive black
holes that are now ``dead''.

Distant Galaxies:

Some types of galaxies are still forming stars at the present epoch
(e.g. spiral and irregular galaxies). However, the past was marked
by a much higher rate of star formation than the present-day average
rate because there was more gas clouds in the past. Galaxies,
themselves, were built in the past from high, initial rates of star
formation.

The time of quasars is also during the time of first star formation in
galaxies, so the two phenomenon are related, the past was a time of rapid
change and violent activity in galaxies.

Space observations called the Hubble Deep
Field produced of faint galaxies and
distant galaxies at high
redshift which confirmed, quantitatively, our estimates of the
style and amount of star formation. Nature lends a hand by providing
of distant galaxies by gravitational lensing, as seen in this
HST image of CL0024.

Interestingly enough, it is often easier to simulate the evolution of
galaxies in a computer, then use the simulations to solve for various
cosmological constants, such as Hubble's constant or the geometry of
the Universe. The field of extragalactic studies is just such a process
of iteration on the fundamental constants of the Universe and the
behavior of galaxies with time (i.e. galaxy evolution).

The most distant galaxies are studied with a combination of the ground and
space-based telescopes. Ground-based scopes for deep imaging of the light
of distant galaxies, space-based to provide high resolution images of
distant galaxies.

The most distant objects are found by the `dropout' method, where the high
redshift of a galaxies means that the blue portion of its spectrum will
have very little luminosity compared to its red light. The object appears
to disappear or dropout in blue light.

Galaxy Evolution:

The phenomenon of lookback time allows us to actually observe the
evolution of galaxies. We are not seeing the same galaxies as today, but
it is possible to trace the behavior of galaxies types with distance/time.

It is known that galaxies form from large clouds of gas in the early
Universe. The gas collects under self-gravity and, at some point,
the gas fragments into star cluster sized elements where star
formation begins. Thus, we have the expectation that distant
galaxies (i.e. younger galaxies) will be undergoing large amounts of
star formation and producing hot stars = blue stars. The study of
this phenomenon is called color evolution.

Computer simulations also indicate that the epoch right after galaxy
formation is a time filled with many encounters/collisions between
young galaxies. Galaxies that pass near each other can be captured
in their mutual self-gravity and merge into a new galaxy. Note that
this is unlike cars, which after collisions are not new types of
cars, because galaxies are composed of many individual stars, not
solid pieces of matter. The evolution of galaxies by mergers and
collisions is called number evolution.

Thus, our picture of galaxy evolution, incorporating both these principles,
looks like the following: